1. Field of the Invention
This invention relates to a liquid crystal optical element and an optical pickup apparatus.
2. Description of the Related Art
A method is known in which the beam reflected from an optical recording medium is split by a beam splitter and applied to a photodetector, and utilizing the zero-order light and the primary light entering the beam splitter, the FE (focus error) signal and the TE (tracking error) signal are obtained (for example, Patent Document 1).
Patent Document 1, in place of the beam splitter, describes a beam splitter having three types of regions and a photodetector corresponding to the beam splitter. The beam splitter transmits the greater part of the incident beam and generates the zero-order light, and by transmitting a part of the light amount, generates the primary light from each of the three types of the regions. The beam splitter described in Patent Document 1 is a diffraction grating actually formed with a plurality of grooves, and used by being fixedly arranged in the light path of the optical beam.
Incidentally, in the optical pickup apparatus used for both write and read operations on a CD or DVD on the one hand and write and read operations on a Blu-ray disk on the other hand, the red beam (660 nm) for CD or DVD an the blue beam (405 nm) for the Blu-ray disc may share the same light path. In such a case, photodetectors usable for the respective light beams are required, which in turn requires a plurality of corresponding beam splitters for radiating a plurality of split beams onto the photodetectors.
For this purpose, however, a mechanism is required to put a plurality of beam splitters in and out of the beam light path, thereby inconveniently increasing the cost while at the same time making it impossible to reduce the size of the apparatus.
Also, demand has arisen for what is called a liquid crystal lens with variable focal length to reproduce, record and erase information on and from optical recording media different in the thickness of the protective layer such as CD, DVD and Blu-ray disc or an optical recording medium having multilayer recording surfaces using a common objective lens. In the optical magnification variable mechanism used for a conventional digital camera or the like, a lens moving mechanism is essential, which requires both a large space and a high cost. In the liquid crystal lens, on the other hand, no movable parts are required, and therefore, a magnification variable mechanism can be realized both in a small space and at a low cost.
In view of this, a technique is known in which the radius of curvature of the spherical component of the corrugated surface shape of the optical beam is changed using the transparent electrode having a plurality of concentrically arranged band-shaped pattern electrodes in the split spatial light modulator of liquid crystal type (for example, Patent Document 2). The transmission wave surface transmitted through the split spatial light modulator is subjected to the stepped phase modulation by a plurality of concentrically arranged band-shaped pattern electrodes to thereby generate the primary diffraction light. Incidentally, in order for the transmission wave surface passed through the split spatial optical modulator to generate the primary diffraction light, a stepped phase modulation having a phase difference of about λ/2 is required.
By using the split spatial light modulator described in Patent Document 2, the focal point of the objective lens can be substantially changed, and therefore, a plurality of optical recording media or an optical recording medium having a multilayer recording surface can be processed with a common objective lens.
Even by use of a transparent electrode having a plurality of band-shaped pattern electrodes arranged concentrically, however, the stepped phase modulation having a phase difference of about λ/2 is difficult to realize in the entire range of the effective light fluxes.
Patent Document 1: JP-A-2004-281026 (FIGS. 12, 13, 18 and 19)
Patent Document 2: JP-A-2006-338840 (FIG. 2)